Frequency Response

Frequency Response is a measure of the range of frequencies that the system is capable of producing.  For example: 20-20,000 Hz - Range of human hearing.

Highest frequency that can be reproduced equals one-half the SR.

The term for this: Nyquist frequency.  Helps to determine the effect of sampling rate on the quality of the digital audio.

Why?  Remember that pitch has a fundamental PLUS all of the harmonics that make up that pitch.   This needs to be taken into account.

Example:  The eighth harmonic of 4.2 kHz tone would be 33.6 kHz (8 x 4.2 kHz), well beyond the range of the human ear. 

CDs use 44.1 KHz therefore the highest frequency is just over 22 KHz.

Goal is to have the output of a digital audio system equal to or better than the input.

Sampling rates less than 44kHz will produce less than optimum quality to the human ear. 


What happens to frequencies above 22KHz? 
They are filtered out in the conversion process.

Why don’t all devices use the CD standard, or higher?
Higher sampling rates use up more system memory, both for processing and storage.

Variable sampling rates allow the SR to be adjusted according to frequency.

Sample Size (Resolution)

Sample size or bit depth (bit rate) refers to the number of bits used for each individual sample. “Width or memory.”

The greater the number of bits, the higher the resolution
8-bit systems
12-bit systems
16-bit systems (CDs)
24 bit resolution (HD audio with a 96 KHz sampling rate).

Signal-To-Noise Ratio

Signal to noise ratio  (SNR) is a comparison between the level of desired signal and the amount of undesired noise "hiss" present in the sound.

Measured in decibels; the higher the dB level, the better the SNR.

8 bit systems = 48 dB
12 bit systems = 72 dB
16 bit systems = 96 dB (CDs, DAT)

Different Specs

CD Player: FR = 20KHz  S/N=94dB
SR=44.1 w/ 16 bits

Cassette Tape: FR =15KHz  S/N=75dB
Reel-to-Reel: FR=15KHz  S/N=55dB

DVDs   SR=48-96 KHz w/24 bits

Analogy 16 bit 300 dpi = 44.1      
              32 bit 600 dpi=96  quite perceptible

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